Likely cause for this is 3.0.3 getting removed from the "check this every pass" list in the sentry config" when 3.0.4 was added, so when 3.0.4 replaced 3.0.3 on the mozilla-current mirrors, sentry didn't pick up that 3.0.3 was gone yet.

Dave is really asking for a process change here - in bouncer, we should not remove the Check Now bit on the current release when adding entries for a new one. We should wait for a while (a day?, a few hours) when something is removed from the mozilla-current rsync module (/pub/mozilla.org/zz/rsyncd-mozilla-current.exclude) before revoking Check Now.
I'll update our wiki docs for this, and we'll see if sentry completes quickly enough on the next release.

Yes, that's correctly what I was asking for. Sentry needs to keep scanning for the file you just removed until it detects that it's been removed in order to avoid 404s with people getting sent to the mirrors that just replaced it with 3.0.4 because it hasn't figured out that they don't have 3.0.3 yet until the overnight check everything pass runs.